The invention relates to an exhaust-air guide of a fuel cell stack, in particular in a motor vehicle, having a cooling device which belongs to the functional environment of the fuel cell stack and which is in the form of a cooler structure through which ambient air flows. With regard to the prior art, reference is made, by way of example, to DE 10 2008 029 529 A1.
Fuel cells, at least those of PEM type of construction, must be cooled during operation. For this reason, a suitable cooling device must be provided for a so-called fuel cell stack formed by a stack of multiple individual fuel cells. A cooling device of this type is normally formed by a circuit for a heat carrier medium and a heat transfer means, in particular a heat exchanger, in which circulated heat carrier medium, which absorbs heat in the fuel cell stack, releases said received amount of heat to an ambient air flow conducted through the heat transfer means, in particular the heat exchanger, referred to in the present case in generalized form as cooler structure. Also known, however, are air-cooled fuel cell stacks which themselves form or include a suitable cooler structure past which or through which an ambient air flow is conducted for cooling purposes, in this regard, cf. for example the as yet unpublished German patent application having reference number 102012206459.1. Here, in most cases, for the delivery of the ambient air flow through the cooler structure—be it an independent heat transfer means (in particular a heat exchanger) or a cooler structure integrated in the fuel cell stack—an independent delivery device (blower, fan, etc.) is required, which in some cases has a relatively high energy requirement.
The problem addressed by the present invention is that of specifying an improvement in this regard.
The solution to the problem consists in an exhaust-air guide of a fuel cell stack, in particular in a motor vehicle, having a cooling device which belongs to the functional environment of the fuel cell stack and which is in the form of a cooler structure through which ambient air flows, wherein at least a part of the exhaust air of the fuel cell stack is conducted into the cooler structure, or downstream of the cooler structure as viewed in the throughflow direction of the ambient air through the cooler structure, such that the exhaust-air flow gives rise, at the cooler structure, to an increase in the mass flow of the ambient air through the cooler structure in accordance with the jet pump principle (specifically in relation to the ambient air mass flow that prevails, with otherwise unchanged boundary conditions, without the approach according to the invention of conducting fuel cell exhaust air to the cooler structure). In other words, the exhaust-air flow conducted in suitable fashion to the cooler structure is intended to give rise, at the cooler structure and in accordance with the jet pump principle, to a pressure gradient which at least partially delivers the ambient air through the cooler structure.
According to the invention, the exhaust air of the fuel cell stack—in the case of PEM fuel cells, this is the cathode exhaust-air flow—is at least partially conducted to the cooler structure such that the pressure or positive pressure of the exhaust-air flow is utilized for the delivery of ambient air as cooling air which flows through the cooler structure. Here, the fundamentally known jet pump principle (known in particular from a suction jet pump formed by a suitable merging of pipes) is utilized. Here, it is expressly pointed out that, on a cooler structure according to the invention, it is by no means imperative for the fuel cell exhaust air and the ambient air to be explicitly merged in a pipe structure; rather, it suffices for the exhaust air of the fuel cell stack to be suitably conducted into the cooler structure or downstream of the cooler structure (as viewed in the throughflow direction of the ambient air through the cooler structure), for example in multiple pipes oriented at least approximately parallel to the inflow surface of the cooler structure, from which pipes the exhaust air emerges via outlet openings in the wall of said pipes (“pipe wall”). The outlet openings are oriented at a suitable angle with respect to the throughflow direction of the (desired) cooling air flow through the cooler structure. At all times, it is possible in the proposed way for the pressure potential that exists in the exhaust-air flow of the fuel cell stack to be utilized for the delivery of ambient air and thus of cooling air through the cooler structure, in such a way that the exhaust-air flow gives rise to an increase of the mass flow of ambient air compared with the ambient air mass flow that prevails, for otherwise unchanged boundary conditions, without the approach according to the invention of conducting fuel cell exhaust air to the cooler structure.
As regards the fuel cell exhaust air conducted in accordance with the invention, or the exhaust-air flow, the pressure potential or positive pressure thereof results from the prior delivery of ambient air as reaction air into the fuel cell stack, which ambient air reacts there in a known manner on one of the electrodes—on the cathodes in the case of a PEM fuel cell—with the fuel flow (in particular in the form of hydrogen) that is conducted to the other side of the respective cathode-electrolyte-anode unit, before the ambient air is subsequently discharged from the fuel cell stack as an exhaust-air flow. Normally, the temperature and, in particular, the moisture content of the exhaust air of the fuel cell stack is increased in relation to the ambient air that is initially supplied, as reaction air, to the fuel cells. In an advantageous refinement of the invention, it is therefore possible for the exhaust air of the fuel cell stack to be cooled, before being conducted in accordance with the invention to the cooler structure, in a suitable heat transfer means, in particular a heat exchanger (preferably again with the aid of ambient air), wherein moisture advantageously condenses out, such that no fogging as would otherwise normally occur, arises as a result of the exhaust-air flow of the fuel cell stack.
In one advantageous refinement of the present invention, the flow speed and/or flow direction of the fuel cell exhaust-air flow relative to the cooler structure, and thus for example the outflow direction and/or the outflow speed of the exhaust air out of the pipes, and generally from any system by way of which the fuel cell exhaust air is conducted in the manner according to the invention to the cooler structure, may vary in a targeted fashion. For example, for this purpose, the outlet openings in the pipe wall of the pipes may be varied by way of a slide or the like. By means of slides, it is for example possible for the cross-sectional area of the outlet openings to be variable, giving rise to different flow speeds for the fuel cell exhaust-air flow; by way of slides or the like, it is however also possible for the flow direction of the exhaust-air flow relative to the cooler structure to be varied in a desired manner. In the case of the pipes mentioned by way of example, through which the fuel cell exhaust air is conducted into or downstream of the cooler structure, it is however also possible for the one or more pipes themselves to be rotatable about their longitudinal axis, and thus for the flow direction of the exhaust-air flow emerging from the one or more pipes relative to the cooler structure to be varied. If multiple pipes are provided, through which the fuel cell exhaust air is conducted to the cooler structure, it is also possible, in a manner dependent on a wide variety of boundary conditions, for some of the pipes to be, in effect, deactivated, that is to say to not be charged with fuel cell exhaust air, whereby the entire exhaust-air flow is distributed among a smaller number of pipes, and thus the flow speed of the exhaust-air flow in the smaller number of pipes, and consequently also the outlet speed out of the pipe, are increased. At this juncture, it is expressly pointed out that it is by no means imperative for the fuel cell exhaust-air flow to be conducted to the cooler structure via multiple pipes with outlet openings provided in the pipe wall, as is optionally proposed; rather, for this purpose, use may also be made of other air-guiding systems. Here, control of the mass flow distribution by way of suitable geometric adaptations is self-evidently again possible.
If the fuel cell exhaust-air flow is, in the manner according to the invention, conducted at least partially into the cooler structure and the cooler structure is an otherwise conventional vehicle cooler with pipes which conduct a cooling liquid and on which there are provided cooling fins along which the ambient air flows as cooling air, it is possible, in order to realize a particularly compact design, for the pipes which conduct the exhaust-air flow to form, at least in sections, a structural unit with the pipelines that conduct the cooling liquid, for example in the form of a pipe with a partition running along the pipe axis.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.